Our overall aims are to elucidate the biochemical mechanisms underlying the essential role of glucocorticoids (e.g. corticosterone) in the onset and maintenance of lactation and of mineralocorticoids (e.g. aldosterone) in sodium conservation by the lactating mammary gland. The proposed study has a fundamental bearing on lactation and the health of the mother and child. Our specific aims are to determine the pattern and rate of metabolism of tritium-labeled corticosterone and aldosterone by the lactating mammary gland in: (1) In vitro experiments. The labeled adrenocortical steroid hormones are incubated with mammary alveolar cells isolated as intact acini from lactating rats. The rate of uptake of labeled hormone by the intact acini and the distribution of unchanged hormone and radiometabolites among the subcellular fractions of the cell homogenates will be determined. (2) In vivo experiments. The labeled adrenocortical hormones are infused in lactating rats at a constant rate over a prolonged period of time to attain steady state levels of radioactivity in the milk and serum. The precise nature and concentration of the radiometabolites (and also unchanged hormone) in the milk, mammary glands (washed free of milk or serum), and serum will be determined for comparative purposes. The influence of estrogen (which inhibits lactation) and corticosteroid-binding globulin (which regulates the concentration of unbound corticosterone) on adrenocortical steroid metabolism in the in vitro and in vivo experiments will be ascertained.